The present invention relates to electrical connectors and more particularly to a printed circuit board having a plurality of conductive spring connectors for applying a potential difference across a piezoelectric crystal used in a megasonic cleaning system.
It is well-known that sound waves in the frequency range of 0.4 to 2.0 megahertz (MHZ) can be transmitted into liquids and used to clean particulate matter from damage sensitive substrates. Since this frequency range is predominantly near the megahertz range, the cleaning process is commonly referred to as megasonic cleaning. Among the items that can be cleaned with this process are semiconductor wafers in various stages of the semiconductor device manufacturing process, disk drive media, flat panel displays and other sensitive substrates.
Megasonic acoustic energy is generally created by exciting a crystal with radio frequency AC voltage. The acoustical energy generated by the crystal is passed through an energy transmitting member and into the cleaning fluid. Frequently, the energy transmitting member is a wall of the vessel that holds the cleaning fluid. The crystal and its related components are referred to as a megasonic transducer. For example, U.S. Pat. No. 5,355,048, discloses a megasonic transducer comprised of a piezoelectric crystal attached to a quartz window by several attachment layers. The megasonic transducer operates at approximately 850 KHz. Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer in which energy transmitting members comprised of quartz, sapphire, boron nitride, stainless steel or tantalum are glued to a piezoelectric crystal using epoxy.
It is also known that piezoelectric crystals can be bonded to certain materials using indium. For example, U.S. Pat. No. 3,590,467 discloses a method for bonding a piezoelectric crystal to a delay medium using indium where the delay medium comprises materials such as glasses, fused silica and glass ceramic.
In many megasonic transducers of the prior art, the crystal is excited by soldering the active lead from the RF generator to a silver electrode layer that has been applied to the back of the crystal. Several problems exist with this arrangement. First, excessive heat can build up at the point of connection of the active lead to the silver electrode. This heat can cause the silver layer to delaminate from the piezoelectric crystal, thereby causing the transducer to fail. Second, air-backed piezoelectric transducers are designed to provide a uniform forward acoustic field. Solder points on the backside of the piezoelectric crystal can dampen the acoustic properties of the piezoelectric crystal, thereby causing an irregularity in the forward acoustic field. Third, once the active lead has been soldered to the piezoelectric crystal, there is no practical way of replacing a defective crystal without melting the solder.
Briefly, the present invention is a system for making electrical connections to a piezoelectric crystal in a megasonic transducer. The system comprises a printed circuit board member, one or more first spring connectors and one or more second spring connectors. The board member has a first copper trace plated on it that is adapted for electrical connection to the active terminal of an RF generator. A second copper trace is plated on the board member and is adapted for electrical connection to the ground terminal of the RF generator. The first spring connectors are electrically connected to the first trace and are positioned to make electrical contact with the back side of the piezoelectric crystal. The second spring connectors are electrically connected to the second trace and are positioned to make electrical contact with the front side of the piezoelectric crystal.
Each first spring connector comprises a first base button mechanically connected to the board member and electrically connected to the first trace, a first spring mechanically connected to the first base button, and a first top button mechanically connected to the first spring and positioned to electrically contact the back side the piezoelectric crystal. The first base button, the first spring, and the first top button are all electrically conductive.
Similarly, each second spring connector comprises a second base button mechanically connected to the board member and electrically connected to the second trace, a second spring mechanically connected to the second base button, and a second top button mechanically connected to the second spring and positioned to electrically contact the front side of the piezoelectric crystal. The second base button, the second spring, and the second top button are all electrically conductive.
In use, the board member is positioned inside a water-tight housing with the back side of the piezoelectric crystal pressing against the top buttons of the first spring connectors. The top buttons of the second spring connectors press against a conductive layer that is in electrical contact with the front side of the piezoelectric crystal. A coaxial cable connects the RF generator to the board member. Since the first spring connectors are electrically connected to the first trace and the second spring connectors are electrically connected to the second trace, a potential difference is set up across the piezoelectric crystal so as to excite it at the frequency of the RF voltage supplied by the RF generator.